The plasma membrane is composed of both lipids and proteins. The relative concentration of each in the plasma membrane is partly regulated by the

Golgi complex

Explanation:

The Golgi complex, also known as the Golgi apparatus, is a cell organelle of eukaryotic cells that functions to sort different molecules which are destined to the plasma membrane. In the Golgi apparatus, proteins and lipids from the Endoplasmic Reticulum (ER) are subsequently processed and classified to be transported to different cell locations including, among others, the plasma membrane, or vesicles which are secreted from cells by exocytosis. Moreover, this organelle also contains specific enzymes (glycosylation enzymes) that function to modify proteins in order to form glycoproteins.